Imaging element for making a lithographic printing plate wherein that imaging element comprises a thermosensitive mask

ABSTRACT

According to the present invention there is provided an imaging element for making a lithographic printing plate comprising on a support having a hydrophilic surface a photosensitive layer and a thermosensitive layer, said thermosensitive layer being opaque to light for which said photosensitive layer has spectral sensitivity and said thermosensitive layer being capable of rendered transparent upon exposure to laser light characterised in that an intermediate layer being soluble or swellable in an aqueous medium is provided between said photosensitive layer and said thermosensitive layer.

DESCRIPTION

1. Field of the Invention

The present invention relates to an imaging element for making alithographic printing plate wherein that imaging element comprises athermosensitive mask on a photosensitive coating.

2. Background of the Invention

Lithography is the process of printing from specially prepared surfaces,some areas of which are capable of accepting lithographic ink, whereasother areas, when moistened with water, will not accept the ink. Theareas which accept ink form the printing image areas and theink-rejecting areas form the background areas.

In the art of photolithography, a photographic material is madeimagewise receptive to oily or greasy inks in the photo-exposed(negative-working) or in the non-exposed areas (positive-working) on ahydrophilic background.

In the production of common lithographic printing plates, also calledsurface litho plates or planographic printing plates, a support that hasaffinity to water or obtains such affinity by chemical treatment iscoated with a thin layer of a photosensitive composition. Coatings forthat purpose include light-sensitive polymer layers containing diazocompounds, dichromate-sensitized hydrophilic colloids and a largevariety of synthetic photopolymers. Particularly diazo-sensitizedsystems are widely used.

Upon image-wise exposure of the light-sensitive layer the exposed imageareas become insoluble and the unexposed areas remain soluble. The plateis then developed with a suitable liquid to remove the diazonium salt ordiazo resin in the unexposed areas.

Alternatively, printing plates are known that include a photosensitivecoating that upon image-wise exposure is rendered soluble at the exposedareas. Subsequent development then removes the exposed areas. A typicalexample of such photosensitive coating is a quinone-diazide basedcoating.

Typically, the above described photographic materials from which theprinting plates are made are camera-exposed through a photographic filmthat contains the image that is to be reproduced in a lithographicprinting process. Such method of working is cumbersome and laborintensive. However, on the other hand, the printing plates thus obtainedare of superior lithographic quality.

Attempts have thus been made to eliminate the need for a photographicfilm in the above process and in particular to obtain a printing platedirectly from computer data representing the image to be reproduced. Inparticular it has been proposed to coat a silver halide layer on top ofthe photosensitive coating. The silver halide can then directly beexposed by means of a laser under the control of a computer.Subsequently, the silver halide layer is developed leaving a silverimage on top of the photosensitive coating. That silver image thenserves as a mask in an overall exposure of the photosensitive coating.After the overall exposure the silver image is removed and thephotosensitive coating is developed. Such method is disclosed in forexample JP-A 60-61752 but has the disadvantage that a complexdevelopment and associated developing liquids are needed.

GB 1.492.070 discloses a method wherein a metal layer or a layercontaining carbon black is provided on a photosensitive coating. Thismetal layer is then ablated by means of a laser so that an image mask onthe photosensitive layer is obtained. The photosensitive layer is thenoverall exposed by UV-light through the image mask. After removal of theimage mask, the photosensitive layer is developed to obtain a printingplate. This method however still has the disadvantage that the imagemask has to be removed prior to development of the photosensitive layerby a cumbersome processing.

Systems are also known where a mask is image-wise formed on aphotosensitive layer by transfer of a masking substance to thephotosensitive coating, e.g. by means of laser transfer or xerography asdisclosed in EP-A 1138. However, such method is generally slow and maynot meet the required image resolutions for obtaining high qualityprints.

3. Summary of the Invention

It is an object of the present invention to provide a method for makingprinting plates of high quality in a convenient way and wherein certaindisadvantages of the prior art are removed.

In accordance with the present invention there is provided an imagingelement for making a lithographic printing plate comprising on a supporthaving a hydrophilic surface a photosensitive layer and athermosensitive layer, said thermosensitive layer being opaque to lightfor which said photosensitive layer has spectral sensitivity and saidthermosensitive layer being capable of rendered transparent uponexposure to laser light characterised in that an intermediate layerbeing soluble or swellable in an aqueous medium is provided between saidphotosensitive layer and said thermosensitive layer.

The present invention further discloses a method for obtaining alithographic printing plate using an imaging element as defined above.

4. Detailed Description of the Invention

Thanks to the use of an intermediate layer that is soluble or swellablein an aqueous alkaline medium the following advantages are realised.Firstly, adverse effects of the thermosensitive layer on properties ofthe photosensitive layer such as developability thereof are avoided.Further, the sensitivity of the thermosensitive layer towards laserexposure can be improved and in particular there is no need to separatethe thermosensitive layer from the imaging element prior to developmentsince it is easily removed during development of the photosensitivelayer thanks to the presence of the intermediate layer. The presentinvention will now be described in more detail without the intention tolimit the invention to any of the embodiments described hereinafter.

a. Imaging element

a.1. Thermosensitive layer

A thermosensitive layer for use in connection with the present inventioncomprises an infrared pigment dispersed in a binder. A particularlydesirable infrared pigment is carbon black. However, other pigments canbe used such as e.g. a conductive polymer particle, metal carbides,borides, nitrides, carbonitrides, bronze-structured oxides and oxidesstructurally related to the bronze family but lacking the A componente.g. WO₂.9.

As a binder for the thermosensitive layer, any binder can be usedhowever, polymeric binders are especially preferred. Preferred polymericbinders include aqueous swellable or soluble binders in particular thosethat are swellable or soluble in an alkaline aqueous medium. Examples ofsuitable binders are e.g. the hydrophilic polymers mentioned hereinafterfor use in the intermediate layer and/or the alkali soluble bindersmentioned in the composition of the photosensitive layer.

It is further particularly desirable that a thermosensitive layer inthis invention further includes a thermodegradable polymer in particularone that decomposes exothermally. In the latter case, sensitivity of thethermosensitive layer is improved. A particular example of athermodegradable polymer that decomposes exothermally is anitrocellulose.

A thermosensitive layer in connection with the present invention mayfurther be cross-linked to make the imaging element less prone to damagecaused during handling of the imaging element.

A thermosensitive layer in the present invention should also be opaqueto light for which the photosensitive layer has spectral sensitivity.Generally used photosensitive layers and in particular those preferredin this invention are UV sensitive. A thermosensitive layer for usetherewith should therefore be opaque to UV-light. According to apreferred embodiment, this may be accomplished by using an infraredpigment that also shows a substantial absorption for UV-light. Aparticularly preferred compound in this respect is carbon black.However, in accordance with an alternative embodiment, thethermosensitive layer may include a UV-absorbing compound capable ofbeing ablated upon exposure to laser light in addition to a compound Acapable of converting laser light into heat. Said compound A can also bepresent in a layer adjacent to said thermosensitive layer. Examples ofsuch UV-absorbing compounds are UV absorbing dyes or pigments. SuitableUV absorbing masking dyes have a sufficient high extinction coefficientin the wavelength range from 300 to 450 nm so that with a reasonableamount of masking dye, the amount of UV light penetrating through thethermosensitive layer is not more than 10% and more preferably not morethan 1% and most preferably not more than 0.1%. Examples of suitableUV-masking dyes are UVINUL D49, UVINUL E50 and UVINUL N539 from BASF,TINUVIN P from Ciba-Geigy, INTRAWITE OB and INTRAWITE YELLOW 2GLN fromCrompton and Knowles Ltd., 4-dimethylaminobenzophenone,4-phenylazophenol etc. The compound A is preferably an infraredabsorbing compound and the laser light is preferably infrared laserlight. More preferably said infrared absorbing compound is an infraredabsorbing dye or a conductive polymer.

The thickness of the thermosensitive layer in connection with thepresent invention is preferably chosen such that sufficient opaquenessof the layer is obtained (preferably 90% or more of the light to whichthe photosensitive layer is responsive is absorbed by thethermosensitive layer, more preferably 99%, most preferably 99.9%) whilemaintaining a suitable sensitivity. Typically, the thickness of thethermosensitive layer ranges for example from 0.1 μm to about 4 μm andmore preferably between 0.5 μm and 1.5 μm

A thermosensitive layer in connection with the present invention mayfurther include additional other components to realise various otherdesired functionalities such as e.g. visual inspection that may berealised by including a colored dye in the thermosensitive layer.

a.2. Intermediate layer

In accordance with the present invention, the imaging element preferablyalso includes an intermediate layer between the photosensitive layer andthe thermosensitive layer. Such intermediate layer should be soluble orswellable in an aqueous medium preferably an aqueous alkaline medium.Suitable intermediate layers preferably contain a hydrophilic binder.Suitable hydrophilic binders are e.g. polyvinyl alcohol,polyvinylpyrrolidon, polyethyleneoxide, celluloses, sacharides, gelatin,carboxyl containing polymers such as e.g. homo- or copolymers of(meth)acrylic acid, maleinic acid anhydride based polymers, polymerscontaining phenolic hydroxy groups e.g. polyvinylphenols etc. Anintermediate layer in connection with the present invention may containvarious additional components e.g. a compound capable of convertinglaser light into heat may be desirably added to the intermediate layer.

The dry thickness of the intermediate layer is preferably comprisedbetween 0.5 and 5 μm, more preferably between 1 and 3.5 μm.

a.3. Photosensitive layer

A photosensitive layer in accordance with the present invention maycomprise any suitable light-sensitive composition from which an inkaccepting image on the hydrophilic surface of the support of the imagingelement can be obtained. Examples of such light-sensitive compositionsused herein are those comprising diazo compounds; those comprising azidecompounds as disclosed in U.K. Patent Nos. 1,235,281 and 1,495,861;those containing photo-crosslinkable photopolymers as disclosed in U.S.Pat. Nos. 4,072,528 and 4,072,527 and in particular those described inmore detail below.

Among these light-sensitive compositions, preferably used are thosecomprising diazo compounds since they are synthetically excellent invarious properties such as storage properties of the photosensitivelayer; developability, for instance, development latitude; imageproperties, e.g. quality of images; printing properties, e.g., inkreceptivity and wear resistance; and low possibility of causingenvironmental pollution of developers used.

The light-sensitive compositions containig diazo compounds can roughlybe divided in two groups, i.e. negative-working type andpositive-working type ones.

The negative-working light-sensitive compositions containing diazocompounds comprise light-sensitive diazo compounds and preferablypolymeric compounds. As the light-sensitive diazo compounds there may beused any ones conventionally known and preferred examples thereof aresalts of organic solvent-soluble diazo resins such as salts ofcondensates of p-diazodiphenylamine and formaldehyde or acetaldehydewith hexafluorophosphates and salts thereof with2-hydroxy-4-methoxybenzophenone-5-sulfonic acid salts.

On the other hand, examples of the foregoing polymeric compoundspreferably used are copolymers of acrylic acid or methacrylic acid;crotonic acid copolymers; itaconic acid copolymers, maleic acidcopolymers, cellulose derivatives having carboxyl groups on the sidechains thereof, polyvinyl alcohol derivatives having carboxyl groups onthe side chains thereof, copolymers of hydroxyalkyl (meth)acrylatehaving carboxyl groups on the side chains thereof, and unsaturatedpolyester resins having carboxyl groups.

As the diazo compounds used in a positive-working light-sensitivecomposition, any compounds conventionally known may be utilized andtypical examples thereof are o-quinonediazides and preferablyo-naphthoquinonediazide compounds. Particularly preferred areo-naphthoquinonediazidosulfonic acid esters or o-naphthoquinonediazidocarboxylic acid esters of various hydroxyl compounds; ando-naphthoquinonediazidosulfonic acid amides oro-naphthoquinonediazidocarboxylic acid amides of various aromatic aminecompounds. Examples of such phenols include phenol, cresol, resorcin andpyrogallol; examples of such carbonyl group-containing compounds areformaldehyde, benzaldehyde and acetone. Examples of preferred hydroxylcompounds include phenol-formaldehyde resin, cresol-formaldehyde resin,pyrogallol-acetone resin and resorcin-benzaldehyde resin.

Typical examples of o-quinonediazide compounds include esters ofbenzoquinone-(1,2)-diazidosulfonic acid ornapthoquinone-(1,2)-diazidosulfonic acid and phenol-formaldehyde resinor cresol-formaldehyde resin; ester ofnaphthoquinone-(1,2)-diazido-(2)-5-sulfonic acid and pyrogallol-acetoneresin as disclosed in U.S. Pat. No. 3,635,709; and ester ofnaphthoquinone-(1,2)-diazido-(2)-5-sulfonic acid andresorcin-pyrogallol-acetone copolycondensates as disclosed in J.P. KOKAINo. Sho 55-76346.

Examples of other useful o-quinonediazide compounds are polyestershaving hydroxyl groups at their termini esterified witho-napthoquinonediazidesulfonyl chloride as disclosed in J.P. KOKAI No.Sho 50-117503; homopolymers of p-hydroxystyrene or copolymers thereofwith other copolymerizable monomers esterified witho-naphtoquinonediazidosulfonyl chloride as disclosed in J.P. KOKAI No.Sho 50-113305; ester of bisphenol-formaldehyde resin ando-quinonediazidosulfonic acid as disclosed in J.P. KOKAI No. Sho54-29922; condensates of alkyl acrylate-acryloyoxyalkylcarbonate-hydroxyalkyl acrylate copolymers witho-naphthoquinonediaziosulfonyl chloride as disclosed in U.S. Pat. No.3,859,099; reaction products of copolymerized products of styrene andphenol derivatives with o-quinonediazidosulfonic acid as disclosed inJ.P. KOKOKU No. Sho 49-17481; amides of copolymers of p-aminostyrene andmonomers copolymerizable therewith and o-naphthoquinonediazidosulfonicacid or o-naphthoquinonediazidocarboxylic acid as disclosed in U.S. Pat.No. 3,759,711; as well as ester compounds of polyhydroxybenzophenonefand o-naphthoquinonediazidosulfonlyl chloride.

These o-quinonediazide compounds may be used alone, but are preferablyused as a mixture with an alkalki-soluble resin to form alight-sensitive layer.

Preferred alkali-soluble resins include novolak type phenol and typicalexamples thereof are phenolformaldehyde, cresol-formaldehyde resin, andphenol-cresol-formaldehyde copolycondensed resins as disclosed in J.P.KOKAI No. Sho 55-57841. More preferably, the foregoing phenol resins aresimultaneously used with a condensate of phenol or cresol substitutedwith an alkyl group having 3 to 8 carbon atoms and formaldehyde such ast-butylphenol-formaldehyde, as described in J.P. KOKAI No. Sho50-125806.

Moreover, it is also possible to optionally incorporate, into thelight-sensitive composition, alkali-soluble polymers other than theabove listed alkali-soluble novolak phenolic resins. Examples of suchpolymers are styrene-acrylic acid copolymer, methylmethacrylate-methacrylic acid copolymer, alkali-soluble polyurethaneresin, and alkali-soluble vinylic resins and alkali-soluble polybutyralresins as disclosed in J.P. KOKOKU No. Sho 52-28401.

The amount of the o-quinonediazide compounds is preferably 5 to 80% byweight and more preferably 10 to 50% by weight based on the total weightof the solid contents of the light-sensitive composition. On the otherhand, that of the alkali-soluble resins is preferably 30 to 90% byweight and more preferably 50 to 85% by weight based on the total weightof the solid contents of the light-sensitive composition.

A photosensitive layer in connection with this invention may be appliedin the form of a multilayered structure. Moreover, the light-sensitivecomposition in the photosensitive layer or multilayer package mayfurther comprise optional components such as dyes, plasticizers andcomponents for imparting printing-out properties (ability of providing avisible image immediately after imagewise exposure).

The coated amount of a photosensitive layer applied onto the hydrophilicsurface of a support preferably ranges from 0.1 to 7 g/m² and morepreferably 0.5 to 4 g/m².

Preferred photo-crosslinking materials in the present invention arebased on photo-crosslinking polymers having a maleimido group at theirside chain. In order to elevate their photosensitivity sensitisers areadded such as thioxanthones, benzophenone, Michler's ketone,anthraquinones, anthracene, chrysene, p-dinitrobenzene, 2-nitrofluorene,as well as sensitizers described in JP-A-62-294238, JP-A-2-173646,JP-A-2-236552, JP-A-3-54566 and JP-A-6-107718. (The term "JP-A" as usedherein means an "unexamined published Japanese patent application".)

A photo-crosslinking polymer having a maleimido group at its sidechain(s) includes, for example polymers that are described in, U.S. Pat.No. 4,079,041 (corresponding to JP-A-52-988); West German Patent2,626,769; European Patents 21,019 and 3,552; Die AngewandteMakromolekulare Chemie, 115 (1983), pp. 163-181; JP-A-49-128991 toJP-A-49-128993, JP-A-50-5376 to JP-A-50-5380, JP-A-53-5298 toJP-A-53-5300, JP-A-50-50107, JP-A-51-47940, JP-A-52-13907,JP-A-50-45076, JP-A-52-121700, JP-A-50-10844, JP-A-50-45087,JP-A-58-43951; West German Patents 2,349,948 and 2,616,276.

Of these polymers, those having, at their side chains, two or moremaleimido groups on average in one molecule and having a mean molecularweight of 1000 or more are preferably used in the present invention.

An imaging element containing any of these polymers is preferablydeveloped with an aqueous alkaline developer substantially notcontaining an organic solvent, in view of the environmental safety.Thereore, it is preferable that these polymers are soluble in orswellable with aqueous alkalis. Accordingly, it is preferably thatmonomers having a maleimido group at its side chain are copolymerizedwith monomers having a alkali-soluble group to obtain these polymers.

As the alkali-soluble group, preferred are acid groups having a pKa of14 or less. Specific examples of such monomers having an alkali-solublegroup include vinyl monomers having a carboxyl group, such as acrylicacid, methacrylic acid, maleic acid, itaconic acid; vinyl monomershaving a --CONHSO₂ -group; vinyl monomers haivng an --SO₂ NH-group;vinyl monomers having a phenolic hydroxyl group; vinyl monomers having aphosphoric acid group or a phosphonic acid group; as well as maleicanhydride, and itaconic anhydride.

The alkali-soluble group having monomer and the maleido group havingmonomer are generally copolymerized at a ratio of from 10/90 to 70/30 bymol, preferably from 20/80 to 60/40 by mol, to easily give aphoto-crosslinking polymer for use in the present invention. The polymerhas preferably an acid value of from 30 to 500, especially preferablyfrom 50 to 300.

Of such photo-crosslinking polymers, especially useful are copolymerscomposed of an N- 2-methacryloyloxy)alkyl!-2,3-dimethylmaleimide andmethacrylic acid or acrylic acid, such as those described in DieAngewandte Makromolekulare Chemie, 128 (1984), pp. 71-91. In producingthese copolymers, vinyl monomers may additionally copolymerized, as thethird component, to easily give polynary copolymers as desired. Forinstance, alkyl methacrylates or alkyl acrylates of which homopolymershave a glass transition temperature not higher than room temperature canbe copolymerized as the third component to give flexible copolymers.

A photo-crosslinking polymer for use in the present invention haspreferably a weight average molecular weight of 1000 or more, morepreferably from 10,000 to 500,000, and even more preferably from 20,000to 300,000.

As mentioned above, it is desirable to add a sensitizer to aphotosensitive layer having a photocross-linkable polymer as describedabove. Preferred is a triplet sensitizer having an absorption peak forensuring sufficient light absorption at 300 nm or more.

Examples of such sensitizer include benzophenone derivatives,benzanthrone derivatives, quinones, aromatic nitro compounds,naphthothiazoline derivatives, benzothiazoline derivatives,thioxanthones, naphthothiazole derivatives, ketocoumarin compounds,benzothiazole derivatives, naphthofranone compounds, pyrylium salts, andthiapyrylium salts.

Specific examples thereof include Michler's ketone,N,N'-diethylaminobenzophenone, benzanthrone,(3-methyl-1,3-diazo-1,9-benz)anthronepicramide, 5-nitroacenaphthene,2-chlorothioxanthone, 2-isopropylthioxanthone, dimethylthioxanthone,methylthioxanthone-1-ethyl carboxylate, 2-nitrofluorenone,2-dibenzoylmethylene-3-methylnaphthothiazoline,3,3-carbonyl-bis-(7-diethylaminocoumarin), 2,4,6-triphyenlthiapyryliumperchlorate, 2-(p-chlorobenzoy)naphthothiazole, as well as sensitizersdescribed in Jp-B-45-8832, JP-A-52-129791, JP-A-62-294238,JP-A-2-173646, JP-A-2-131236, European Patent 368,327, JP-A-2-236552,JP-A-3-54566 and JP-A-6-107718.

Of these, preferred are the sensitizers described in JP-A-2-236552,JP-A-3-54566 and JP-A-6-107718; and especially preferred are thesensitizers having one or more of --COOH, --NHSO₂ R²⁰, --CONHCOR²⁰and/or --CONHSO₂ R²⁰ (where R²⁰ represents an alkyl group, an aromaticgroup or an alkyl-aromatic group) as an alkali-soluble group in onemolecule, described in JP-A-6-107718.

The amount of sensitizer in the photosensitive layer is convenientlyfrom 1 to 20% by weight, preferably from 2 to 15% by weight, and morepreferably from 3 to 10% by weight, of the total amount of thecompositions in the photosensitive layer.

In addition to the photocross-linkable polymers described above it maybe desirable to add a diazo resin to the photosensitive layer. Examplesof diazo resins include co-condensates composed of aromatic diazoniumcompounds and aldehydes. Specific examples thereof include the diazoresins described in JP-B-49-48001, JP-B-50-7481, JP-B-5-2227,JP-A-3-2864, JP-A-3-240061, and JP-A-4-274429.

Of these, preferred are diazo resins having a carboxyl group in themolecule, for example, diazo resins obtained by co-condensation witharomatic compounds having at least one carboxyl group, such as thosedescribed in JP-A-3-240061, and diazo resins obtained by condensationwith aldehydes having a caboxyl group, such as those described inJP-A-2864.

The amount of the diazo resin in the photosensitive layer is preferablyfrom 0.1 to 30% by weight, preferably from 0.5 to 10% by weight, andmore preferably from 1 to 5% by weight of the total amount of thecompositions in the photosensitive layer.

Further, there may be added a polymer having one or more polymerisablegroups such as the polymers disclosed in JP-B-3-63740, U.S. Pat. Nos.3,376,138, 3,556,793.

The above-mentioned polymer having one or more polymerisable groups ispreferably soluble in or swellable with aqueous alkaline developers,like the polymers having a maleimido group at the side chain. Therefore,this polymer is preferably a copolymer composed of one or more monomershaving an alkali-soluble group, e.g. as mentioned above.

a.4. Support

In accordance with the present invention, an imaging element comprises asupport having a hydrophilic surface. Suitable supports for use in thisinvention are e.g. metal supports in particular grained and anodisedaluminium or supports comprising a substrate such as e.g. paper orplastic film provided with a hydrophilic layer, preferably a hydrophiliclayer that is cross-linked.

A particularly suitable cross-linked hydrophilic layer may be obtainedfrom a hydrophilic binder cross-linked with a cross-linking agent suchas formaldehyde, glyoxal, polyisocyanate or a hydrolysedtetra-alkylorthosilicate. The latter is particularly preferred.

As hydrophilic binder there may be used hydrophilic (co)polymers such asfor example, homopolymers and copolymers of vinyl alcohol, acrylamide,methylol acrylamide, methylol methacrylamide, acrylic acid, methacrylicacid, hydroxyethyl acrylate, hydroxyethyl methacrylate or maleicanhydride/vinylmethylether copolymers. The hydrophilicity of the(co)polymer or (co)polymer mixture used is preferably the same as orhigher than the hydrophilicity of polyvinyl acetate hydrolyzed to atleast an extent of 60 percent by weight, preferably 80 percent byweight.

The amount of crosslinking agent, in particular of tetraalkylorthosilicate, is preferably at least 0.2 parts by weight per part byweight of hydrophilic binder, preferably between 0.5 and 5 parts byweight, more preferably between 1.0 parts by weight and 3 parts byweight.

A cross-linked hydrophilic layer in accordance with the presentinvention preferably also contains substances that increase themechanical strength and the porosity of the layer. For this purposecolloidal silica may be used. The colloidal silica employed may be inthe form of any commercially available water-dispersion of colloidalsilica for example having an average particle size up to 40 nm, e.g. 20nm. In addition inert particles of larger size than the colloidal silicacan be added e.g. silica prepared according to Stober as described in J.Colloid and Interface Sci., Vol. 26, 1968, pages 62 to 69 or aluminaparticles or particles having an average diameter of at least 100 nmwhich are particles of titanium dioxide or other heavy metal oxides. Byincorporating these particles the surface of the cross-linkedhydrophilic layer is given a uniform rough texture consisting ofmicroscopic hills and valleys, which serve as storage places for waterin background areas. The thickness of a cross-linked hydrophilic layermay vary in the range of 0.2 to 25 μm and is preferably 1 to 10 μm.

Further particular examples of suitable cross-linked hydrophilic layersfor use in accordance with the present invention are disclosed in EP-A601240, GB-P-1419512, FR-P-2300354, U.S. Pat. No. 3,971,660, U.S. Pat.No. 4,284,705 and EP-A 514490.

As substrate on which the hydrophilic layer is provided it isparticularly preferred to use a plastic film e.g. substratedpolyethylene terephthalate film, cellulose acetate film, polystyrenefilm, polycarbonate film etc. The plastic film support may be opaque ortransparent.

It is particularly preferred to use a polyester film support to which anadhesion improving layer has been provided. Particularly suitableadhesion improving layers for use in accordance with the presentinvention comprise a hydrophilic binder and colloidal silica asdisclosed in EP-A 619524, EP-A 620502 and EP-A 619525. Preferably, theamount of silica in the adhesion improving layer is between 200 mg perm² and 750 mg per m². Further, the ratio of silica to hydrophilic binderis preferably more than 1 and the surface area of the colloidal silicais preferably at least 300 m² per gram, more preferably a surface areaof 500 m² per gram.

b. Method for making a lithographic printing plate

b.1 image-wise exposure

Image-wise exposure in connection with the present invention involvesthe use of a laser emitting in the infrared (IR), i.e. emitting in thewavelength range above 700 nm, preferably 700-1500 nm. Particularlypreferred for use in connection with the present invention are laserdiodes emitting around 830 nm (gallium-arsenide laser diodes) or aNdYAG-laser emitting at 1060 nm.

A preferred imaging apparatus suitable for image-wise exposure inaccordance with the present invention preferably includes a laser outputthat can be provided directly to the imaging elements surface via lensesor other beam-guiding components, or transmitted to the surface of ablank imaging element from a remotely sited laser using a fiber-opticcable. A controller and associated positioning hardware maintains thebeam output at a precise orientation with respect to the imagingelements surface, scans the output over the surface, and activates thelaser at positions adjacent selected points or areas of the imagingelement. The controller responds to incoming image signals correspondingto the original document and/or picture being copied onto the imagingelement to produce a precise negative or positive image of thatoriginal. The image signals are stored as a bitmap data file on acomputer. Such files may be generated by a raster image processor (RIP)or other suitable means. For example, a RIP can accept Input data inpage-description language, which defines all of the features required tobe transferred onto the imaging element, or as a combination ofpage-description language and one or more image data files. The bitmapsare constructed to define the hue of the color as well as screenfrequencies and angles in case of amplitude modulation screening.However, the present invention is particularly suitable for use incombination with frequency modulation screening as disclosed in e.g.EP-A 571010, EP-A 620677 and EP-A 620674.

The imaging apparatus for use in the present invention is preferablyconfigured as a flatbed recorder or a drum recorder with the imagingelement mounted to exterior cylindrical surface of the drum. In apreferred drum configuration, the requisite relative motion between thelaser beam and the imaging element is achieved by rotating the drum(andthe imaging element mounted thereon) about its axis and moving the beamparallel to the rotation axis, thereby scanning the imaging elementcircumferentially so the image "grows" in the axial direction.Alternatively, the beam can move parallel to the drum axis and, aftereach pass across the imaging element, increment angularly so that theimage on the imaging element "grows" circumferentially. In both cases,after a complete scan by the beam and development, an imagecorresponding to the original will have been applied to the surface ofthe imaging element. In the flatbed configuration, the beam is drawnacross either axis of the imaging element, and is indexed along theother axis after each pass. Of course, the requisite relative motionbetween the beam and the imaging element may be produced by movement ofthe imaging element rather than (or in addition to) movement of thebeam.

Regardless of the manner in which the beam is scanned, it is generallypreferable (for reasons of speed) to employ a plurality of lasers andguide their outputs to a single writing array. The writing array is thenindexed, after completion of each pass across or along the imagingelement, a distance determined by the number of beams emanating from thearray, and by the desired resolution (i.e. the number of image pointsper unit length).

b.2. overall exposure

Overall exposure in connection with the present invention is carried bymeans of a light source that emits at least in the wavelength range forwhich the photosensitive layer of an imaging element in connection withthe present invention has spectral sensitivity. In a practicalembodiment in this invention, the photosensitive layer is UV-sensitiveand/or optionally sensitive to the short wavelength part of the visiblespectrum e.g. upto green. Overall exposure may for example be carriedout by exposure sources such as high or medium pressure halogen mercuryvapour lamps, e.g. of 1000 W.

b.3. development

Development in connection with the present invention is carried out by asuitable liquid capable of removing either the exposed or non-exposedareas of the photosensitive layer. The appropriate composition of adeveloping liquid in the present invention will depend on the kind ofphotosensitive layer and preferably is such that during development, thethermosensitive layer and optional intermediate layer are removed at thesame time.

For ecological reasons, it is highly preferred that an aqueous baseddeveloping liquid is used preferably without additional organicsolvents. A particularly preferred developing liquid is an aqueousalkaline liquid.

Particularly suitable developing liquids for use with the preferredphotosensitive coatings described above are as follows.

A developer preferably used in the invention is an aqueous solutionsmainly composed of alkali metal silicates and alkali metal hydroxides.As such alkali metal silicates, preferably used are, for instance,sodium silicate, potassium silicate, lithium silicate and sodiummetasilicate. On the other hand, as such alkali methal hydroxides,preferred are sodium hydroxide, potassium hydroxide and lithiumhydroxide. From the viewpoint of preventing the formation of insolubleprecipitates, it is particularly desirable that the developer compriseat least 20 mole % of potassium with respect to the total amount ofalkali metals in the aqueous developing solution.

The developers used in the invention may simultaneously contain otheralkaline agents. Examples of such other alkaline agents include suchinorganic alkaline agents as ammonium hydroxide, sodium tertiaryphosphate, sodium secondary phosphate, potassium tertiary phosphate,potassium secondary phosphate, ammonium tertiary phosphate, ammoniumsecondary phosphate, sodium bicarbonate, sodium carbonate, potassiumcarbonate and ammonium carbonate; and such organic alkaline agents asmono-, di- or triethanolamine, mono-, di- or trimethylamine, mono-, di-or triethylamine, mono-, di- or isopropylamine, n-butylamine, mono-, di-or triisopropanolamine, ethyleneimine, ethylenediimine andtetramethylammonium hydroxide.

It is particularly desirable that the molar ratio of silicate SiO₂ ! toalkali metal oxide M₂ O! in a developer for use with this invention isfrom 0.6 to 1.5, preferably 0.7 to 1.3. In addition, the concentrationof SiO₂ in the replenisher preferably ranges from 2 to 4% by weight.

In a developer used in the present invention, it is possible tosimultaneously use organic solvents having solubility in water at 20° C.of not more than 10% by weight according to need. Examples of suchorganic solvents are such carboxilic acid esters as ethyl acetate,propyl acetate, amyl acetate, benzyl acetate, ethylene glycol monobutylacetate, butyl lactate and butyl levulinate; such ketones as ethyl butylketone, methyl isobutyl ketone and cyclohexanone; such alcohols asethylene glycol monobutyl ether, ethylene glycol benzyl ether, ethyleneglycol monophenyl ether,, benzyl alcohol, methylphenylcarbinol, n-amylalcohol and methylamyl alcohol; such alkyl-substituted aromatichydrocarbons as xylene; and such halogenated hydrocarbons as methylenedichloride and monochlorobenze. These organic solvents may be used aloneor in combination. Particularly preferred is benzyl alcohol in theinvention. These organic solvents are added to the developer generallyin an amount of not more than 5% by weight and preferably not more than4% by weight.

A developer used in the present invention may simultaneously contain asurfactant for the purpose of improving developing properties thereof.Examples of such surfactants include salts of higher alcohol (C₈ ˜C₂₂)sulfuric acid esters such as sodium salt or lauryl alcohol sulfate,sodium salt of octyl alcohol sulfate, ammonium salt of lauryl alcoholsulfate, Teepol B-81 (trademark available from Shell Chemicals Co.,Ltd.) and disodium alkyl sulfates; salts of aliphatic alcohol phosphoricacid esters such as sodium salt of cetyl alcohol phosphate; alkyl arylsulfonic acid salts such as sodium salt of dodecylbenzene sulfonate,sodium salt of isopropylnapthalene sulfonate, sodium salt ofdinaphthalene disulfonate and sodium salt of metanitrobenze sulfonate;sulfonic acid salts of alkylamides; and sulfonic acid salts of dibasicaliphatic acid esters such as sodium dioctyl sulfosuccinate and sodiumdihexyl sulfosuccinate. These surfactants may be used alone or incombination. Particularly preferred are sulfonic acid salts. Thesesurfactants may be used in an amount of generally not more than 5% byweight and preferably not more than 3% by weight.

In order to enhance developing stability of the developers the followingcompounds may simultaneously be used.

Examples of such compounds are neutral salts such as NaCl and KBr asdisclosed in J.P. KOKAI No. Sho 58-75152; chelating agents such as EDTAand NTA as disclosed in J.P. KOKAI No. Sho 58-190952 (U.S. Pat. No.4,469,776); complexes such as Co(NH₃)₆ !Cl₃ as disclosed in J.P. KOKAINo. Sho 59-121336 (U.S. Pat. No. 4,606,995); ionizable compounds ofelements of the group IIa, IIIa or IIIb of the Periodic Table such asdisclosed in J.P. KOKAI No. Sho 55-25100; anionic or amphotericsurfactants such as sodium alkyl naphthalene sulfonate andN-tetradecyl-N,N-dihydroxyethyl betaine as disclosed in J.P. KOKAI No.Sho 50-51324; tetramethyldecyne diol as disclosed in U.S. Pat. No.4,374,920, nonionic surfactants as disclosed in J. P. KOKAI No. Sho60-213943; cationic polymers such as methyl chloride quaternary productsof p-dimethylaminomethyl polystyrene as disclosed in J.P. KOKAI No. Sho55-95946; amphoteric polyelectrolytes such as copolymer of vinylbenzyltrimethylammonium chloride and sodium sulfites as disclosed in J.P.KOKAI No. Sho 56-142528; reducing inorganic salts such as sodium sulfiteas disclosed in J.P. KOKAI No. Sho 57-192952 (U.S. Pat. No. 4,467,027)and alkaline-soluble mercapto compounds or tioether compounds such asthiosalicylic acid, cysteine and thioglycolic acid; inorganic lithiumcompounds such as lithium chloride as disclosed in J.P. KOKAI No. Sho58-95444; organic lithium compounds such as lithium benzoate asdisclosed in Japanese Patent Publication for Opposition Purpose(hereinafter referred to as J.P. KOKOKU) No. Sho 50-34442;organicometallic surfactants cotaining Si, Ti or the like as disclosedin J.P. KOKAI No. Sho 59-75255; organoboron compounds as disclosed inJ.P. KOKAI No. Sho 59-84241 (U.S. Pat. No. 4,500,625); quaternaryammonium salts such as tetraalklammonium oxides as disclosed in EuropeanPatent No. 101,010; and bacterides such as sodium dehydroacetate.

In accordance with a method of the present invention the imaging elementis wiped with a cleaning means, preferably a dry cleaning means e.g. acotton pad or a paper towel after the image-wise exposure and before theoverall exposure is effected. This is particularly preferred when thethermosensitive layer comprises carbon black or a metallic pigment.

In accordance with the present invention the imaging element can also beprocessed after mounting the image-wise and overall exposed imagingelement on a print cylinder of a printing press. This is especiallysuitable for imaging elements which can be processed by water or awater-alcohol solution. Such imaging elements are described in EP-A631189 which therefor is incorporated herein by reference. According toa preferred embodiment, the printing press is then started and while theprint cylinder with the imaging element mounted thereon rotates, thedampener rollers that supply dampening liquid are dropped on the imagingelement and subsequent thereto the ink rollers are dropped. Generallyafter about 10 revolutions of the print cylinder the first clear anduseful prints are obtained.

According to an alternative method, the ink rollers and dampener rollersmay be dropped simultaneously or the ink rollers may be dropped first.

Suitable dampening liquids that can be used in connection with thepresent invention are aqueous solutions generally having an acidic pHand comprising an alcohol such as isopropanol. With regard to dampeningliquids useful in the present invention, there is no particularlimitation and commercially available dampening liquids, also known asfountain solutions, can be used.

According to an alternative method, the imaging element is first mountedon the print cylinder of the printing press and then image-wise andoverall exposed directly on the press. Subsequent to exposure, theimaging element can be developed as described above. Suitable imagingelements for an image-wise exposure on the press are said imagingelements which can be developed while mounted on the print cylinder ofthe printing press as described above.

The present invention will now be further illustrated by way of thefollowing examples without any intention to limit the invention thereto.All parts are by weight unless stated otherwise.

EXAMPLE

Preparation of a Lithographic Base.

To 440 g of a dispersion containing 21.5% TiO₂ (average particle size0.3 to 0.4 μm) and 2.5% polyvinyl alcohol in deionized water weresubsequently added, while stirring, 250 g of a 5% polyvinyl alcoholsolution in water, 105 g of a hydrolyzed 22% tetramethylorthosilicateemulsion in water and 12 g of a 10% solution of a wetting agent.

To this mixture was added 193 g of deionized water and the pH wasadjusted to pH=4.

The obtained dispersion was coated on a polyethylenterephthalate filmsupport (coated with a hydrophilic adhesion layer) to a wet coatingthickness of 50 g/m², dried at 30° C., and subsequently hardened bysubjecting it to a temperature of 57° C. for 1 week. To this base wasfurther provided an aqueous solution of (pH=5) of Dormacid (a dextranmodified with a diethylaminoethyl group available from Pfeifer & Langen)and a cationic wetting agent to a dry coating thickness of 30 mgDormacid per m². The obtained element was then heated for 1 week at 57°C.

Preparation of the Imaging Element.

An imaging element according to the invention was produced by preparingthe following light-sensitive compositions.

Preparation of the UV-Sensitive Coating.

To 63 g of a 20% dispersion of polymethylmethacrylate (particle diameterof 40 nm) stabilized with cetyltrimethylammoniumbromide in deionizedwater was subsequently added, while stirring, 120 g of a 5% solution ofa 98% hydrolized polyvinylacetate, having a weight average molecularweight of 200 000 g/mol (MOWIOL 56-98 available from Hoechst), in waterand 15 g of a dispersion containing 10% of Heliogen Blue D 7565(available from BASF) and 5% polyvinylalcohol (MOWIOL 56-98) in water.46 g of a 15% solution of the condensation product of diphenylaminediazonium salt and formaldehyde (NEGALUX N18 available from PCAS) and 20g of a 15% solution of the condensation product of methoxy-diphenylaminediazonium salt and formaldehyde (DIAZO No. 8 available from Fairmount)in water was then slowly added. Finally 30 g of a 1.6% solution ofcationic fluor containing surfactant (Fluorad FC135 available from 3 M)in water, and 726 ml of water were added. The UV-sensitive compositionwas coated to the above described lithographic base in an amount of 35g/m² (wet coating amount) and then dried at 30° C.

On this UV-photosensitive coating a polyethyleneglycol layer was coated.This layer was coated from a 4% solution of polyethyleneglycol (ANTAROXE4000 commercially available from GAF) in an amount of 20 g/m² (wetcoating amount).

Preparation of the IR-Sensitive Coating.

The IR-sensitive formulation contains the following ingredients in partsby weight, as indicated.

    ______________________________________    Methyl-ethyl ketone        834.5    Nitrocellulose E620 (available from Wolff Walsrode)                               25.5    (UV-absorber)              25    (IR-dye 1)                 10    (IR-dye 2)                 50    (Cyan-dye 1)               7.5    (Cyan-dye 2)               14.5    (Magenta-dye 1)            11    (Magenta-dye 2)            6.5    (Yellow-dye 1)             8    (Yellow-dye 2)             7.5    ______________________________________    UV-absorber    1 #STR1##    IR dye 1    2 #STR2##    IR dye 2    3 #STR3##    Cyan dye 1    4 #STR4##    Cyan dye 2    5 #STR5##    Magenta dye 1    6 #STR6##    Magenta dye 2    7 #STR7##    Yellow dye 1    8 #STR8##    Yellow dye 2    9 #STR9##    ______________________________________

The UV-sensitive layer was overcoated by means of a knife coater withthe IR-sensitive formulation to a wet coating thickness of 20 μm.

Preparation of a Printing Plate and Making Copies of the Original

An imaging element as described above was subjected to a scannnig Nd YLFinfrared laser emitting at 1050 nm (scan speed 1.1 m/s, spot size 15 μmand 400 mW power on the surface of the imaging element). After thisexposure the IR-sensitive mask had disappeared in areas exposed to thelaser-beam.

Subsequently, the imaging element was exposed to a high pressure halogenmercury vapour lamp of 1000 W at a distance of 70 cm for 90 s.

Further the imaging element was subjected to a developing process withwater, hereby removing the non-image parts.

After development, the imaging element was mounted on a GTO 46offsetpress. As an ink was used K+E 123 W and as a fountain solutionRotamatic. Printing was started and a good printing quality was obtainedwithout any ink uptake in the non-image parts.

Example 2

On a Ozasol N61 plate was coated a 2.5 wt % aqueous solution ofpolyvinylalcohol (MOWIOL 56-98) in an amount of 40 g/m² (wet coatingamount) and then dried at 40° C.

On top of this polyvinylalcohol layer an IR-sensitive mask as describedin example 1 was coated.

After laser-exposure the IR-sensitive mask had (as in example 1)disappeared in areas exposed to the laser-beam.

Subsequently, the imaging element was exposed to a high pressure halogenmercury vapour lamp of 1000 W at a distance of 70 cm for 50 s.

Further the imaging element was subjected to a developing process withthe aqueous alkaline developer Fuji DN-5, hereby removing the non-imageparts.

After development, the imaging element was mounted on a GTO 46offsetpress. As an ink was used K+E 123 W and as a fountain solutionRotamatic. Printing was started and a good printing quality was obtainedwithout any ink uptake in the non-image parts.

We claim:
 1. An imaging element for making a lithographic printing platecomprising on a support having a hydrophilic surface a photosensitivelayer and a thermosensitive layer, said thermosensitive layer beingopaque to light for which said photosensitive layer has spectralsensitivity and said thermosensitive layer being capable of renderedtransparent upon exposure to laser light wherein an intermediate layerbeing soluble or swellable in an aqueous medium is provided between saidphotosensitive layer and said thermosensitive layer.
 2. An imagingelement according to claim 1 wherein said photosensitive layer isUV-sensitive and said thermosensitive layer is opaque to UV-light.
 3. Animaging element according to claim 2 wherein said thermosensitive layercomprises carbon black.
 4. An imaging element according to claim 2wherein said thermosensitive layer includes a UV absorbing dye capableof being ablated upon said exposure to laser light and rendering saidthermosensitive layer opaque to UV-light and wherein saidthermosensitive layer or an adjacent layer thereto includes a compound Acapable of converting said laser light to heat.
 5. An imaging elementaccording to claim 4 wherein said compound is an infrared absorbingcompound and said laser light is infrared laser light.
 6. An imagingelement according to claim 5 wherein said infrared absorbing compound isan infrared absorbing dye or a conductive polymer.
 7. An imaging elementaccording to claim 1 wherein said thermosensitive layer comprises athermally degradable polymer.
 8. An imaging element according to claim 1wherein said photosensitive layer comprises an alkali soluble or alkaliswellable copolymer of a maleimide.
 9. An imaging element according toclaim 1 wherein said photosensitive layer comprises a diazo resin or adiazonium compound.
 10. An imaging element according to claim 1 whereinsaid intermediate layer is soluble or swellable in an aqueous alkalinemedium.
 11. A method for making a lithographic printing plate comprisingthe steps of:image-wise exposing an imaging element as defined in claim1 by means of a laser thereby image-wise rendering said thermosensitivelayer transparent to light for which said photosensitive layer hasspectral sensitivity; overall exposing a thus obtained imaging elementwith light for which said photosensitive layer has spectral sensitivity;developing a thus obtained imaging element thereby removing saidintermediate layer, thermosensitive layer and either the non-exposed orthe exposed areas of said photosensitive layer so as to leave an inkaccepting image of said photosensitive layer on said support.
 12. Amethod according to claim 11 wherein said imaging element is mounted ona drum and said image-wise exposure is an internal or external drumexposure.
 13. A method according to claim 12 wherein said laser is aninfrared laser.
 14. A method according to claim 11 wherein saidimage-wise exposure is carried out by means of an array of infraredlaser diodes.
 15. A method according to claim 11 wherein said imagingelement is wiped with a cleaning means after the image-wise exposure andbefore the overall exposure is effected.
 16. A method according to claim11 wherein said imaging element is processed after mounting theimage-wise and overall exposed imaging element on a print cylinder of aprinting press.
 17. A method according to claim 11 wherein said imagingelement is image-wise and overall exposed while being mounted on theprint cylinder of a printing press.